1. Field of the Invention
The present invention relates to a lithographic printing plate precursor.
2. Description of the Related Art
Lithographic printing typically involves the use of a so-called printing master such as a printing plate which is mounted on a cylinder of a rotary printing press. The master carries a lithographic image on its surface and a print is obtained by applying ink to said image and then transferring the ink from the master onto a receiver material, which is typically paper. In conventional lithographic printing, ink as well as an aqueous fountain solution (also called dampening liquid) are supplied to the lithographic image which consists of oleophilic (or hydrophobic, i.e. ink-accepting, water-repelling) areas as well as hydrophilic (or oleophobic, i.e. water-accepting, ink-repelling) areas. In so-called driographic printing, the lithographic image consists of ink-accepting and ink-abhesive (ink-repelling) areas and during driographic printing, only ink is supplied to the master.
Lithographic printing masters are generally obtained by the image-wise exposure and processing of a radiation sensitive layer on a lithographic support or by image-wise jetting of ink onto a lithographic support. Imaging and processing renders the so-called lithographic printing plate precursor into a printing plate or master. Imaging-wise exposure of the radiation sensitive coating to heat or light, typically by means of a digitally modulated exposure device such as a laser, triggers a (physico-)chemical process, such as ablation, polymerization, insolubilization by cross-linking of a polymer or by particle coagulation of a thermoplastic polymer latex, solubilization by the destruction of intermolecular interactions or by increasing the penetrability of a development barrier layer. Although some plate precursors are capable of producing a lithographic image immediately after exposure, the most popular lithographic plate precursors require wet processing since the exposure produces a difference of solubility or of rate of dissolution in a developer between the exposed and the non-exposed areas of the coating. In positive working lithographic plate precursors, the exposed areas of the coating dissolve in the developer while the non-exposed areas remain resistant to the developer. In negative working lithographic plate precursors, the non-exposed areas of the coating dissolve in the developer while the exposed areas remain resistant to the developer. Most lithographic plate precursors contain a hydrophobic coating on a hydrophilic support, so that the areas which remain resistant to the developer define the ink-accepting, printing areas of the plate while the hydrophilic support is revealed by the dissolution of the coating in the developer at the non-printing areas. Generation of a lithographic printing master by image-wise jetting of ink on a lithographic support does not require a wet processing during which coating has to be dissolved in a developer. The ink receiving layer, which acts as the image recording layer, can be the surface of an anodised aluminium lithographic support optionally with a post-anodic treatment or coated with a hydrophilic layer. The jetted ink has ink-accepting properties on the press and reveals the printing areas.
In a typical industrial plate making process, the lithographic coating is applied on a support which is provided to the coating head as a web. After coating and drying, the web is sliced (cutting in the web direction) and cut (across the web) to produce individual printing plate precursors. The plates are then stacked, packed and shipped. Typically, the slicing and cutting produces upward burr, i.e. the support at the plate edges is protruding in the direction of the image recording layer, as shown in FIG. 1. In FIG. 1, the image recording layer is represented by the thin line on top of the cross-section of the lithographic printing plate precursor. During shipping, the burr often creates scratches on the surface of both sides of the plate, due to slight shifting of the plates in the stack. Another problem caused by prolonged stacking of plates is blocking, i.e. sticking of adjacent plates. Scratches or scuffs can also occur when plates are removed from the stack, either produced by burr or by the separation of sticking plates. Scratches in the image recording layer at the front side of the plate often produce visible defects in the image area.
In order to reduce the sticking of stacked plates and to avoid scratches during transport and/or plate removal, plate manufacturers have traditionally inserted paper sheets as interleaf between the stacked plates. Unfortunately, however, there are many disadvantages associated with the use of these interleaf sheets. In particular, digital plate exposure systems (commonly referred to as platesetters) are increasingly equipped with automatic plate-feeding devices and consequently, the presence of interleaf sheets creates serious complications. Such devices are designed to mechanically remove a plate from a stack and load it onto the exposure device, while additional machinery is required in order to mechanically remove the interleaf sheets. Moreover, these interleaf sheets represent an additional cost, incl. for their disposal. Therefore there is an increasing desire for the elimination of interleaf sheets in stacks of lithographic printing plate precursors.
The printing plate precursor described in JP 02/040657 A manages to be produced without the use of interleaf sheets. A UV-cured layer produced from a photopolymerisable material is located on the back side of the aluminium support. In EP 1834802 A an infrared sensitive lithographic printing plate precursor comprises on one side of a support an image recording layer and on the other side a back coat layer having a Vickers hardness of 0.2 or less. DE 19908529 A describes a printing plate precursor comprising a pigmented light-sensitive layer on one side of its support and a layer comprising an organic polymeric material having a glass transition temperature of 45° C. or above on the other side.
A lithographic plate precursor which can be stacked without the use of interleaf sheets is also described in EP 528395 A. It comprises a support made of aluminium, a layer of an organic polymeric material having a glass transition temperature of not less than 20° C. with a thickness of from 0.01 to 8.0 μm on the back side of the support and a light sensitive layer on the front side of the support. A discontinuous matting layer consisting of particles is provided on top of the light sensitive layer. The function of the matting agents is to reduce the sticking of stacked plates by allowing air inclusion between the plates. However, matting layers easily damage the image recording layer upon relative movement of stacked plates, because the matting agents are typically inorganic pigment particles of a high hardness. In addition, matting layers, in particular those comprising a particulate material having a low glass transition temperature, tend to stick to the back of the overlying plate in the stack. Besides the interaction between the matting agent and the image recording layer, additional technical problems occur in the production of pigmented back coatings. One of the disadvantages is that the solid constituents sediment very quickly, meaning that additional measures are required in order to be able to coat a homogeneous back layer.
EP1019254A discloses a lithographic printing plate precursor wherein the back side of the support is coated with an inert coating comprising a discontinuous layer obtained by coating a dispersion or spraying a powder. In EP1217447A a precursor material for the production of offset printing plates is disclosed with a continuous pigment free layer on the back side, essentially consisting of an organic polymeric material having a glass transition temperature of at least 45° C. and having a smoothness of from 5 to 800 s. In the same document, applying the back coat via spraying is disclosed. A disadvantage of spraying methods is the low yield since up to 50% of the atomised coating solution may be wasted into the environment. Furthermore there is a high risk of contaminating the manufacturing equipment.
In EP1239328A a precursor material for the production of offset printing plates is disclosed with a continuous electrically conductive back coating. The back layer is coated with a hot-melt ink comprising UV curable organic polymer with a gravure printing technique by means of engraved rollers. The surface of the back layer is smooth or structured.
EP1156370A describes a production method of a back coating of an offset plate which is light sensitive. The back coating is not subjected to image wise exposure. The invention makes it possible to cut a stack of printing plate precursors without considerable cutting burrs and damage to the knife.
Further, as disclosed in JP 62-19315A, burr is generated at the surface side of the radiation sensitive layer of a plate precursor during cutting of the precursor web. Several attempts have been made to reduce or prevent this burr in the manufacturing process, such as trimming corners of the edge portions of the support member with a file or a knife. However, printing plate precursors have to be taken out one by one to be trimmed. Therefore, this method is not appropriate for use in industrial manufacturing.
In the printing plate precursor manufacturing process, printing plate precursors often have to be re-cut after they have been stacked. Re-cutting of plate stacks is usually done with small size adaptations (typically 5 mm or less) of the plate size or plates have to be re-sized from for example an 8-up size to a typical 4-up size.
None of the prior art documents which disclose the use of back coatings in order to avoid the use of interleafs, provides a solution for the problem that, in a stack of plates, the upward burr (i.e. burr protruding towards the image recording side of the plate) which is produced by the slicing and cutting of the plates on the manufacturing line, interacts with the coating on the back side of the adjacent plate during shipping or prolonged storage. Upward burr even occurs when the cutting is carried out from the image recording side of the plate. In a stack of plates, the burr may anchor into the back coating of the adjacent plate increasing stickiness between the plates, which often leads to plate separation problems in the automatic plate loader of the platesetter.
Moreover, none of the prior art documents provides a solution for the problem that, when a stack of plates without interleaf has to be re-cut, the very strong contact between the burr produced by the high pressure during the re-cutting and the back coating of the adjacent plate results in the sticking of the plates to each other at the edges. It is therefore highly desirable to provide a solution to avoid sticking of lithographic printing plate precursors after re-cut of a stack of plates and still providing, without the need for interleaf sheets, sufficient protection of the image recording layer against mechanical damage.